A wireless portable Internet is a next generation communication scheme for further supporting mobility for short range data communication schemes which use fixed access points, such as the conventional wireless LAN.
Various standards for the wireless portable Internet have been proposed, and the international standard of the portable Internet has progressed by focusing on the IEEE 802.16e.
FIG. 1 shows a brief diagram of the wireless portable Internet.
A wireless portable Internet system comprises an SS (subscriber station) 10, base stations 20 and 21 for performing wireless communication with the SS 10, routers 30 and 31 connected to the base stations through a gateway, and the Internet.
The wireless LAN such as the conventional IEEE 802.11 provides a data communication scheme for allowing short-range wireless communication with reference to a fixed access point, and it does not provide mobility of the SS but rather it supports the short-range data communication in a wireless manner instead of on the cable basis.
The wireless portable Internet system driven by the IEEE 802.16 group guarantees mobility and provides a seamless data communication service when the SS 10 shown in FIG. 1 is moved to a cell managed by the base station 21 from another cell managed by the base station 20.
The IEEE 802.16 basically supports the MAN (metropolitan area network), and represents an information communication network covering an intermediate area of between the LAN and the WAN.
Therefore, the wireless portable Internet system supports a handover of the SS 10 in a like manner of the mobile communication service, and assigns dynamic IP addresses according to movement of the SS.
In this instance, the SS communicates with the base stations 20 and 21 through the OFDMA (orthogonal frequency division multiple access) method, which is a modulation and multiple access scheme having combined the OFDM (orthogonal frequency division multiplexing) scheme which uses a plurality of subcarriers of orthogonal frequencies as a plurality of subchannels, and the FDMA (frequency division multiple access) scheme. The OFDMA scheme is essentially resistant to the fading phenomenon generated on the multi-paths, and has high data rates.
Also, the IEEE 802.16 has adopted the AMC (adaptive modulation and coding) scheme for adaptively selecting a modulation and coding scheme according to a request and an acceptance between the SS 10 and the base stations 20 and 21.
FIG. 2 shows a hierarchical structure of the wireless portable Internet system.
The hierarchical structure of the wireless portable Internet system of the IEEE 802.16e is generally classified as a physical layer L10, and an MAC (media access control) layer L21, L22, and L23.
The physical layer L10 performs wireless communication functions executed on the conventional physical layers, such as modulation/demodulation, and encoding/decoding.
The wireless portable Internet system does not have layers classified according to their functions, but allows a single MAC layer to perform various functions, differing from the mobile cellular system.
Regarding sublayers according to the functions, the MAC layer comprises a privacy sublayer L21, an MAC common part sublayer L22, and a service specific convergence sublayer L23.
The service specific convergence sublayer L23 performs a payload header suppression function and a QoS mapping function in the case of consecutive data communication.
The MAC common part sublayer L22, which is the core part of the MAC layer, performs a system access function, a bandwidth allocation function, a connection establishment and maintenance function, and a QoS management function.
The privacy sublayer L21 performs a device authentication function, a security key exchange function, and a data encryption function. Device authentication is performed by the privacy sublayer L21, and user authentication is performed by an upper layer (not illustrated) of the MAC
FIG. 3 shows a brief diagram of a connection configuration between a BS (base station) and an SS in the wireless portable Internet system.
The MAC layer of the SS and the MAC layer of the BS have a connection C1 therebetween.
The phrase connection C1 in the present invention represents not a physically connected relation but rather a logically connected relation, and it is defined to be a mapping relation between MAC peers of the SS and the BS in order to transmit traffic of a single service flow.
Therefore, parameters or messages defined with respect to the connection C1 represent the functions between the MAC peers, and in reality, the parameters or the messages are processed, are converted into frames, and are transmitted through the physical layers, and the frames are parsed and the functions which correspond to the parameters or the messages are executed on the MAC layer.
The MAC messages transmitted through the connection C1 basically comprise a CID (connection identifier) which is an MAC layer address for identifying connections; an MAP for defining a symbol offset and a subchannel offset of bursts time-divided by the SS in a downlink/uplink, and a number of symbols and a number of subchannels of allocated resources; and a channel descriptor for describing characteristics of the physical layer according to the characteristics of the downlink/uplink (a downlink channel descriptor will be referred to as a DCD and an uplink channel descriptor will be referred to as a UCD hereinafter).
In addition, the MAC messages comprise various messages for performing a request (REQ) function, a response (RSP) function, and an acknowledgement (ACK) function on various operations.
FIG. 4 shows a diagram for a frame structure of the wireless portable Internet system.
Frames are classified as a downlink sub-frame F1 and an uplink sub-frame F2 depending on transmission directions. The vertical axis of the frame represents subchannels including orthogonal frequencies, and the horizontal axis denotes a time-divided symbol axis.
The downlink sub-frame F1 comprises a preamble, a downlink MAP, an uplink MAP, and a plurality of downlink bursts. The downlink bursts may not represent the channels or resources classified according to users, but they are classified according to transmission levels with the same modulation scheme or channel encoding.
Therefore, the downlink MAP has offset information, modulation method information, and coding information on a plurality of users who use the same modulation method and channel coding, and allocates the resources to the users. Accordingly, the MAP has a feature of broadcast channels and requires strong robustness.
In the case of the uplink sub-frame F2, transmission is performed per user, and the uplink bursts have per-user information.
FIG. 5 shows a flowchart for establishing a connection process in the wireless portable Internet system.
When an SS enters a base station's area in step S1, the SS establishes downlink synchronization with the BS in step S2. When the downlink synchronization is established, the SS acquires an uplink parameter in step S3. For example, the parameter includes a channel descriptor message according to the physical layer characteristics (e.g., useable burst profiles corresponding to the appropriate SNR (signal to noise ratio) levels).
A ranging process between the SS and the base station is performed in step S4. The ranging process for correcting timing, power, and frequency information between the SS and the base station performs an initial ranging process and a periodic ranging process after the initial ranging.
When the ranging process is finished, a negotiation on basic capabilities for establishing connection between the SS and the base station is performed in step S5. When the negotiation on basic capabilities is finished, the SS is authenticated in step S6 by using a device identifier including an MAC address and a certificate of the SS to the base station.
When the authentication for the SS is finished and a authorization on the wireless portable Internet is confirmed, a device address of the SS is registered in step S7, and an IP address is provided to the SS from an IP address management system such as a DHCP or a Mobile IP and an IP connection is accordingly established in step S8.
The SS assigned with the IP address performs a connection-establishment process for data transmission in step S9.
The above-described wireless portable Internet system not only performs communication near a fixed location but also has mobility in the metropolitan level differing from the conventional wireless LAN communication systems, and hence, batteries are usually used to supply power to the SS. Therefore, the duration of the batteries is a major limitation of the usage time in the wireless portable Internet system.
Therefore, the wireless portable Internet system such as the IEEE 802.16e has proposed a sleep mode for reduction of battery power consumption. The sleep mode is a method for allowing a terminal to enter a sleep state during a sleep interval, and reduce the SS's power consumption when no data to be transmitted to the SS is provided. After entering the sleep state, the SS performs no operation for data transmission during the sleep interval.
The SS is switched to a listening state each time the sleep interval is terminated, and it checks whether data which stands by to be transmitted (to the corresponding terminal) during the sleep interval are provided.
FIG. 6 shows a signal flowchart for a sleep mode operation in the wireless portable Internet system.
Entering the sleep mode by the SS requires a permission by the base station. The SS 10 which attempts to enter the sleep mode establishes a sleep interval to request a sleep mode from the base station 20 in step S10.
When receiving the sleep mode request, the base station assigns a sleep interval to transmit a sloop mode approval to the SS in step S11.
When receiving the sleep mode approval, the SS enters the sleep mode for receiving no data at the sleep mode entering time M in step S12. When the initial sleep interval is passed, the SS is switches to a listening mode to check whether data addressed to the SS (in a transmission standby state) are buffered from the base station during the sleep interval in step S13.
In this instance, when no data addressed to the SS (in the transmission standby state) are buffered during the initial sleep interval, the base station 20 establishes a message for notifying existence of data traffic to be “0” and transmits it to the SS in step S14.
When it is determined that no data traffic is transmitted during the listening mode, the SS enters the sleep mode again in step S15. In this instance, the sleep interval can be established to be equal to or greater than the initial sleep mode.
When data in the transmission standby state with respect to the SS 10 are provided during a second sleep interval, the base station can buffer the data traffic in step S17, and existence of the buffered data are reported in the listening mode of the SS in step S18.
When checking that the data traffic to be transmitted to the SS 10 are found in the listening mode step S16, the SS 10 terminates the sleep mode, enters an awake mode to receive the buffered data traffic, and performs data communication with the base station 20.
The SS 10 proceeds to the sleep mode according to the sleep mode operation when there are no data to be transmitted, thereby preventing unnecessary power consumption.
FIGS. 7 and 8 show exemplified sleep intervals in the sleep mode.
FIG. 7 shows exemplified terminals operable by a power saving operation mode with a periodic sleep mode wherein a subscriber station SS1 listens to a frame once for each N/4 frame, and a subscriber station SS2 listens to a frame once for each N/2 frame.
Therefore, broadcast information which is needed to be listened to by the subscriber states SS1 and SS2 is broadcast once for each N/2 frame, and information which is needed to be transmitted for a specific subscriber station SS1 is broadcast by a subframe with a period of an N/4 frame.
However, the periodic power saving mode is easy to manage, but its power saving efficiency is not good because most of the data traffic is shown at a specific time (i.e., a burst characteristic), and periodic switching to the listening mode is inefficient for power saving in the data communication system such as the Internet.
FIG. 8 shows a power saving mode operation with an exponentially increasing sleep interval.
Since the data traffic other than voice traffic has a burst characteristic and a long-range dependence as described above, it is desirable to exponentially increase the next sleep interval when no data traffic in the transmission standby is provided in the listening mode.
As described, a subscriber station SS3 initially has a sleep interval of an N frame, and it exponentially increases the sleep interval such as to 2N, 4N, and 8N.
However, the case of exponentially increasing the sleep interval is effective when the data traffic has a long-range dependence, but it increases complexity of the system since it must manage the sleep interval and the listening interval for the respective subscriber stations.
Also, the power saving operation method shown in FIG. 8 is not efficient for traffic which has a very long interval and periodically appears.